Many industrial processes involve the conveyance of process fluid streams by pumps, mixers and other submerged or partially submerged shaft-driven machines. Often these shaft-driven machines must be immersed in chemicals which are so toxic, environmentally sensitive, and/or volatile that vertical tank-mounted mechanisms are preferred so as to preclude the need for submerged tank penetrations that could be sources of uncontrolled process fluid leakage to the environment in the event of connection failures. Additionally, many of these process fluids, because of entrained solids and/or corrosives, are not suitable bearing lubricants, thereby requiring that any bearings that are submerged in the process fluid must be isolated from the process fluid and lubricated by a separate bearing lubricant.
With reference to FIG. 1A, one approach is to mount a motor 1 or other rotation-imparting mechanism on the top 4 of a tank containing the toxic, environmentally sensitive, and/or volatile fluid 3, and allow a shaft 5 to extend vertically downward therefrom into the fluid 3. A housing 14 containing at least one seal 10 and at least one bearing 12 can be extended into the fluid so as to support the shaft 5 near its distal end while protecting the bearing 12 from the fluid 3. An appropriate lubricant 100 can be supplied to the bearing 12 within the housing 5, for example by filling the housing 14 with the lubricant 100 as shown in FIG. 1A.
However, some processes involve process fluids at elevated temperatures exceeding 65° C., creating a substantial limitation to the use of standard, commercially available rolling element bearings 12, due to the likelihood that the combination of load generated heat with heat transferred from the surrounding process fluid will cause the temperature of the bearing 12 to exceed the bearing manufacturer's maximum temperature limitation.
With reference to FIG. 1B, one approach to preventing heat damage to the seals 10 and bearings 12 is to use an integral oil circulation system to remove bearing and seal generated heat by circulating the lubricant 100 to an external reservoir (not shown) for cooling and storage. However, for high temperature processes with process fluids at temperatures above 65° C., very large and costly lubrication systems are required so as to remove the heat at a sufficient rate. Also, removal of heat at the high rate necessary to protect the bearings 12 and mechanical seals 10 may tend to cool the surrounding process fluid and thereby interfere with maintaining a desired temperature within the process.
With reference to FIG. 1C, another approach is to locate all seals 10 and bearings 12 outside of the tank 4, thereby isolating them from the elevated temperature of the process fluid. However, modern dynamic mechanical seals 10 require that the shaft 5 be mechanically stable so as to ensure proper operation of the seal. It is well known to those skilled in the art that to provide a suitable shaft stability for a dynamic mechanical seal 10, it is desirable to have a shaft L3/D4 ratio of less than 50, where L is defined as the overhung shaft length in inches between the axial centerline of the bearing 12 closest to the impeller 25 (inboard bearing) and the axial centerline of the impeller, and D is defined as the diameter in inches of the smallest cross section of the shaft 5 within length L, exclusive of the impeller mounting surface. Note that throughout this document values for L3/D4 are expressed in units of in−1.
The larger the L3/D4 ratio, the more shaft deflection is likely to occur. Such shaft deflection may be generated by any unexpected operating conditions, such as pump cavitations, closed suction or discharge valves, or improper operating conditions (i.e. improper pump selection). The greater the shaft deflection, the greater the wear on the seals 10 and bearings 12 in the system.
Therefore, the most cost effective method to provide the required shaft stability for a dynamic mechanical seal 10 and bearing 12 located outside of the tank 4 is to use a rolling element bearing 12 paired with a shaft having L3/D4 of less than 50, so as to prevent the shaft deflection at the dynamic mechanical seal 10 from exceeding the manufacture's recommendations. One common approach is to employ what is known to those familiar in the art as a cantilever shaft 5. A cantilever shaft pump or mixer uses a shaft 5 with a sufficiently large diameter so as to allow the bearings 12 to be mounted external to the tank with the impeller 25 mounted on the distal end of a shaft section that is cantilevered downward from the support bearings 12 into the tank 4. Packing or mechanical shaft seals 10 are mounted at the cover plate 4 so as to prevent leakage. A cantilevered pump or mixer has the advantage of being able to handle toxic, environmentally sensitive, and/or volatile fluids, with or without suspended solids, at elevated temperatures, without the use of any bearing supports below the tank mounting plate.
However, a disadvantage the cantilever designs is that the bearings 12 and shaft 5 need to be quite large in diameter so as to adequately support the cantilevered mass under load without vibration or excessive movement. These large sizes result in extra cost, and often restrict the ready replacement availability of bearings 12 and seals 10, which can be quite costly and complex due to the toxicity, environmental sensitivity, and/or volatility of the process. The large shaft sizes and bearing sizes typical of the cantilever approach also limit the equipment operation to lower operating speeds than would be available with smaller diameter bearings 12 and mechanical seals 10, due to reduced maximum speed limits inherent to larger diameter bearings 12 and seals 10.
Yet another approach is to use pumps and mixers driven by submersible motors that are capable of handling toxic, environmentally sensitive, and/or volatile fluids, including fluids that may contain solids. These units use oil-lubricated or grease-lubricated rolling element bearings in cooperation with suitable dynamic mechanical seals to isolate the bearings from the process fluid. However, this approach suffers from several disadvantages. Submersible motor-driven pumps and mixers, because of electrical losses, have substantially greater heat removal requirements than mechanisms that immerse only bearings and seals in the process fluid. Also, the repair or replacement of a submersible motor is more costly, more frequently required, and more time consuming than the repair or replacement of a standard electric motor which is externally mounted. In addition, many process fluids contain chemicals that are problematic to the integrity of insulation on submerged electrical cables, which can limit the application of submersible motor-driven pumps as compared to pumps with externally mounted motors.
Therefore, there exists a need for a vertical, tank-mounted shaft and bearing mechanism that can be driven by an externally mounted electric motor or other rotation-imparting device, wherein a load driven by the distal end of the shaft is capable of operating submerged at temperatures exceeding 65° C. in a volatile, environmentally sensitive, and/or toxic process fluid which may include entrained solids, wherein the mechanism includes dynamic mechanical seal(s) which are paired with the shaft with an L3/D4 of less than 50 to prevent excessive deflection of the shaft at the dynamic mechanical seal(s), wherein the bearing(s) and seal(s) are of standard sizes and thereby less costly than those used with cantilevered shaft units, and wherein the lubrication cooling system need not be substantially oversized due to a need to protect the bearing(s) and seal(s) from an excessive heat load due to the surrounding process fluid.